Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 306 AM EDT Fri Jul 17 2020 Valid Jul 17/0000 UTC thru Jul 20/1200 UTC ...See NOUS42 KWNO (ADMSDM) for the status of the upper air ingest... 00Z Final Model Evaluation Including Preferences and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Long wave trough crossing the north central US into eastern Canada through Fri with an accompanying surface cold front... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average There exists little spread among the global models in depicting the mid-level trough and associated surface front moving across the northern tier through today and offshore the Northeast tonight. With little spread among the guidance, a general model blend is preferred. ...Long wave trough crossing the Pacific Northwest today, moving into south central Canada by Sat, then clipping the Northern Plains into Upper Great Lakes by Sun... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend, less weight on the 00Z NAM/CMC Confidence: Average 07Z Update: 12Z non-NCEP suite remains generally well clustered to the ensembles and NCEP members. Note a subtle height discrepancy along the east coast in the core of the upper ridge where the CMC is a bit warmer on D2 and D3. The difference overall is negligible, but with such good agreement otherwise, a little less weight on the CMC seems prudent. Otherwise, no changes to the prior blend. Previous Discussion: As an expansive ridge envelops much of the CONUS, flow atop it and across the northern tier and southern Canada becomes low amplitude and fast. Within this flow, a shortwave dropping into the Pacific Northwest today will race eastward to the Great Lakes by Sunday, dragging a cold front beneath it. With the ridge to the south preventing much amplification of any features, the deeper NAM the latter half of D3 both with its mid-level energy and the surface low which develops along the front seem overdone, and are out of tolerance with the global consensus by the end of the forecast period. Otherwise, there exists little spread other than small amplitude perturbations within the flow, such that a general compromise through 48 hours and a non-NAM blend on D3 seems reasonable. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml This product will terminate August 15, 2020: www.weather.gov/media/notification/pdf2/scn20-58pmdhmd_termination. pdf Weiss